Oct 2 2019

Right, so I was fooled by having qqc2-desktop-style listed as Tier 1 on our main documentation page https://api.kde.org/frameworks/index.html . If it is really Tier 3, then maybe it does not even make sense to make IconThemes and ConfigWidgets optional?

plugin/kquickstyleitem.cpp includes ksharedconfig.h but KConfig was not specified as a link dependency for this target. Previously it was found transitively via KF5ConfigWidgets I guess? Now without those optional dependencies it cannot be built. So KConfig should explicitly be specified as another required KF5 dependency, along with Kirigami.

First of all, if you try to build qqc2-desktop-style on a system with only Qt5 and other Tier 1 frameworks installed, you will get the following cmake configure log (https://paste.ubuntu.com/p/YRjSRWm9g6/):

OS Kubuntu 18.04 LTS
Custom-built Qt 5.14 in ~/prefix514
Other Tier 1 Frameworks also built and installed in ~/prefix514

The patch compiles and works.
I've built master branch and with this patch, and run both with QT_SCALE_FACTOR=2. During connection I chose maximum quality option (local network). The results look identical to me:
master on the left, patched D20790 on the right